Lubricating oils are normally classified in terms of their viscosity at some standard temperature. Equally important is a property known as the viscosity index, which is a widely used and accepted measure of the variation in kinematic viscosity due to changes in the temperatures of a petroleum product between 40.degree. and 100.degree. C. (ASTM D2270-86). For an oil to satisfy viscosity requirements optimally at both extremes of a useful temperature range to which it may be subjected, a high viscosity index is necessary. This property can be controlled to some extent by refining, but in recent years the trend has been towards formation of multi-grade oils of extremely high viscosity indexes in which certain polymer compounds which function as viscosity index improvers are added.
While the viscosity index of synthetic lubricating oils can be usefully modified by the addition of oil-soluble polymeric viscosity index (V.I.) improvers, such an addition can introduce chemical instability to the lubricating compositions.
In the industry there is an ever-increasing demand for lubricating compositions showing good flow at low temperatures, yet possessing adequate viscosity at higher temperatures. The lubrication of engines and gears is usually carried out with multi-grade oils based on mineral lubricating oils whose viscosity/temperature characteristic are influenced by the addition of polymers, such as V.I. improvers, such that the classifying features of the SAE oils for winter and summer use respectfully are combined in a single oil.
The performance of such multi-grade oils based on a mineral oil is highly unsatisfactory for a number of reasons. If the amount of the V.I. improvers, e.g., polyacrylates, polymethacrylates, olefin copolymers, added is to remain within tolerable limits, the additional use of paraffinic base oils is inevitable. Cooling of the oils causes the pour point to be reached as a result of the crystallization of solid paraffins. Although the pour point may be lowered by the addition of pour point depressants, the viscosities in the range between the turbidity point and the pour point remain higher than anticipated for the liquid phase due to the aggregation of crystallizable paraffin components. Distinct differences may be observed between the low temperature viscosity calculated by extrapolation of viscosity measurements made at higher temperatures and the low temperature viscosity as actually measured. This increased viscosity greatly restricts the range of application of such oils.
Moreover, such multi-grade oils containing V.I. improvers are not stable to the action of shearing forces encountered under operating conditions. The resulting decrease in viscosity at all temperatures and reduction of the viscosity index impairs the viscosity/temperature characteristic and the original multi-grade character of the oils may be lost.
Synthetic oils, particularly synthetic hydrocarbons, have become widely accepted as replacements for mineral oils and have proven to be interesting lube bases which can be used in many applications.
Polybutenes are known to the art as synthetic, paraffinic hydrocarbons produced by a simple process from readily available feedstocks. Polybutenes are known to be used as lubricants and are oligomers with molecular weights varying between 300 and 3,000 excluding the very viscous derivatives (molecular weights from 20,000 to 100,000) which are used as V.I. improvers and derivatives of even higher molecular weight which are synthetic rubbers. Polybutenes, unfortunately, exhibit high viscosity and high volatility when compared to other synthetic hydrocarbons of the same molecular weight.
The use of polybutenes in synthetic lubricants is described, for example, in U.S. Pat. Nos. 4,299,714 and 4,031,020 to Sugiura et al. These patents disclose fluid systems containing polybutenes of a molecular weight of 100-500, polyalphaolefins of a molecular weight of 100 to 500, mineral oil and additives. The products of this patent, however, appear to be of too low viscosity (5.5 cSt at 210 degrees F.) for use as lubricating oils in internal combustion engines or diesel engines.
U.S. Pat. No. 4,194,057 to Brankling et al. discloses polymer compositions suitable for uses of viscosity improver additives in lubricating oil compositions which include polybutenes of molecular weight 5,000 to 60,000 to prevent gelling of the viscosity improver additive concentrates. Similarly U.S. Pat. No. 4,620,048 to Ver Strate et al. discloses hydrocarbon solutions which contain polybutenes as viscosity index improvers for mineral fluid oils.
U.S. Pat. No. 3,860,522 to Fischer disclose synthetic lubricants which consist of mixtures of esters of branched-chained dicarboxylic acids and aliphatic alcohols with polymers of butenes which have a molecular weight of 1,200 to 4,500. This patent requires that the polybutenes always be mixed with the synthetic ester lubricants disclosed. The accomplishment of some of the objectives of this patent using PAO of viscosities from 40 to 1000 cSt at 100.degree. C. is disclosed in U.S. Pat. No. 4,956,122 to Watts. However, use of these high viscosity PAO's leads to inferior performance such as in Caterpillar diesel engine tests.
In the publication by Thomas et al., entitled "Polybutenes," Industrial and Engineering Chemistry, Volume 32, No. 3, page 299-304, there is a discussion of the use of polybutenes as additives in the production of various petroleum products such as motor oil to improve the viscosity index of the oil. This publication discloses polybutenes of variable molecular weights and characteristics of such polybutenes including blends thereof with asphalts and paraffin wax.
In the publication by Souillard, "The Use of Polybutenes in Lubrication," Proceedings of the ISLE-ASLE International Conference" 1975, page 724 to 737, polybutenes are disclosed which have a molecular weight of 300-1,000 with viscosities similar to mineral oils. These polybutenes are discussed as being industry lube bases which can be used in many applications.
The present invention is an improvement over prior known lubricating compositions and provides for fully synthetic lubricating base oil compositions which exhibit a high viscosity index to provide lubricants ranging from less viscous to more viscous multi-grade motor oils.